1. Field of the Invention
The present invention relates to wireless telephone systems and, in particular, to multi-line wireless telephone systems having limited power for radio-frequency (RF) communications.
2. Description of the Related Art
The use of telephones and telephone systems, including wireless telephone systems, is widespread. In wireless telephone systems, a cordless or wireless telephone handset unit communicates via either analog or digital radio signals with a base unit, which is typically connected via a standard telephone line to an external telephone network. In this manner, a user may employ the wireless handset to engage in a telephone call with another user through the base unit and the telephone network.
Multi-line wireless telephone systems are also in use in various situations, such as businesses with many telephone users. Such systems employ a handset that communicates with up to N handsets simultaneously, typically with digital communications schemes, such as time division multiple access (TDMA). It is desirable to implement the features of current private branch exchange (PBX) systems in a multi-line wireless telephone system.
Conventional multi-line wireless telephone systems, such as digital 900 MHz systems, typically must work within a specified bandwidth and modulation format, which thus constrains the maximum capacity of the radio-frequency (RF) channel used to transmit signals between the base unit and the operating or active handsets. Such systems are also typically power-limited, for example the 900 MHz RF channel cannot exceed 1 Watt in signal strength. Such systems therefore have range limits determined, in part, by the maximum power allowed.
Digital wireless telephone systems typically employ error-detection and correction schemes, such as FEC (forward error correction). Thus, some of the bits transmitted between handset and base unit carry audio and other forms of data, and some of the bits are devoted to error correction. For example, a given digital audio packet may comprise an audio packet header section (data), an audio data samples section (audio), and an FEC data section (error correction).
One problem that arises in digital wireless telephone systems is that, at the range limits, erratic or worse performance may occur. For example, at the range limits, signal strength for a constant- or limited-power signal falls off with distance, thereby degrading the quality of the received signal and increasing the number of bit errors. This can cause calls to be terminated and/or to adversely affect the quality of the audio and other data reconstructed by the relevant transceiver.
A wireless telephone system comprises a base unit coupleable to one or more external telephone lines and has a base transceiver, and at least one wireless handset. The wireless handset has a handset transceiver for establishing a digital link with the base unit via the base transceiver over a wireless channel, wherein the handset and base unit communicate via the digital link by fixed-size audio packets comprising a plurality of audio data samples and a plurality of error correction bits. The digital link qualities are monitored to determine whether the handset is exceeding a range limit. In response to the determination, a packet structure of subsequent audio packets is changed from a first packet structure to a second packet structure to change the relative number of bits devoted to audio data samples and to error correction bits.
UK Patent Application No. 2 291 570 A, filed Jul. 21, 1995, published Jan. 24, 1996, describes an adaptive error correcting device, in which an error state of a communication channel is measured, and then one of a plurality of error encoding systems is selected depending on this measured error state. In general, an error encoding system is selected in this reference so that, when there is higher error measured, lower quality data is encoded and subjected to the error correction provided by a fixed number of error correction bits, so that only data of higher importance are protected from errors by the fixed number error correction bits, i.e. so that the available error correction bits apply to a smaller number of data bits so that the error correction for these data bits improves.